Error correction device and error correction method

ABSTRACT

The present invention is concerned with an error correction device and method for correcting errors in data items sent on a plurality of channels. An identification unit calculates an exclusive OR value for data items sent on the plurality of channels, and adopts the calculated value as data sent on a separate error correction channel. A signal quality detection unit detects the signal qualities concerning the plurality of channels. If the exclusive OR value is not equal zero, an identifying and processing unit replaces data on a channel whose signal quality is detected to be worst with an exclusive OR value for data items sent on channels other than the channel whose signal quality is worst.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an error correction device andan error correction method for correcting errors in data items sent on aplurality of channels.

[0003] According to the present invention, data items sent on aplurality of channels refer to data items to be handled in diversefields such as: the field of the asymmetric digital subscriber line(ADSL) and symmetric digital subscriber line (SDSL) technologies thatutilize wire communication as a transmission medium; the field of theorthogonal frequency division multiplexing (OFDM) technology thatutilizes radiocommunication as a transmission medium; the field of thewavelength division multiplexing (WDM) technology that utilizes anoptical fiber as a transmission medium and adopts light having aplurality of wavelengths; and the field of the technology for performingparallel recording or reproducing of data using a recording medium.

[0004] 2. Description of the Related Art

[0005] Various types of data transmission systems are already known. Forexample, transmission systems utilizing a power line as a datatransmission line are also known. FIG. 22 shows a power line carriercommunication system that is one type of transmission system utilizingthe power line. There are shown a distribution substation 101, an accessnode 102, a high-tension distribution line 103, a pole transformer 104,a low-tension distribution line 105, a drop wire 106, and house wiring107.

[0006] A high ac voltage of, for example, 6.6 kV is distributed from thedistribution substation 101 to each pole transformer 104 over thehigh-tension distribution line 103. The high voltage is stepped down to100 V or 200 V and is supplied to households or any other users. Thiscauses various kinds of electric equipment coupled to the house wiring107 or various kinds of electric equipment plugged into outlets to startoperating.

[0007] The access node 102 installed in the distribution substation 101and a modem (not shown) incorporated in the pole transformer 104 arelinked by an optical fiber transmission line (not shown). The opticalfiber transmission line is generally laid along the high-tensiondistribution line 103. The modem in the pole transformer 104 transformsa light signal into an electric signal or vice versa. The low-tensiondistribution line 105, drop wire 106, and house wiring 107 are used as adata transmission line for wire communication. Once a terminal isplugged into an outlet coupled to the house wiring 107, a power linecarrier communication system, called a “Last One Mile” system, can beconstructed for communication of data between the access node 102 andthe terminal.

[0008] In this kind of power line carrier communication system, thelow-tension distribution line 105 offers an inductive impedance to theflow of an alternating current from the modem in the pole transformer104, and the drop wire 106 and house wiring 107 offer a capacitiveimpedance thereto. Moreover, various kinds of electric equipment coupledto the house wiring 107 generally have an anti-noise capacitorincorporated therein. Therefore, the impedance to the flow of analternating current from the modem in the pole transformer 104 includesa relatively large inductive reactance and a large capacitive reactance.

[0009] Consequently, for the modem in the pole transformer 104, thelow-tension distribution line 105 is comparable to a low-pass filter. Asignal received by a modem coupled to the house wiring 107 has thehigh-frequency components thereof reduced greatly. Namely, thehigh-frequency components of the received signal are buried under noise.Moreover, the low-frequency components of the received signal do notdecay as much as the high-frequency components do. A random noise sentfrom a switching power supply or an inverter circuit incorporated inelectric equipment is delivered to largely affect the received signal.

[0010] For example, referring to FIG. 23A, the axis of the ordinateindicates power levels PWR and the axis of the abscissa indicatesfrequencies. A dot-line curve expresses a noise-cancelingcharacteristic, and solid-line curves indicate received signal levelsand noise levels. By utilizing the noise-canceling characteristicexpressed with the dot-line curve, the noise level of low-frequencycomponents can be made lower than the received signal level. However, asfar as electric equipment employing an inverter is concerned, noisehaving a comb-like wave are often distributed over a broad frequencyband. In this case, for example, as shown in FIG. 23B, high noise levelsare detected outside a noise-canceling band. This leads to the frequentoccurrence of errors in the received data.

[0011] The orthogonal frequency division multiplexing (OFDM) techniqueis one of the techniques for transmitting data using multiple carriers.The carriers are orthogonal to one another. Multiplexing is performedusing the multiple carriers. It is therefore possible to assignfrequencies, which do not fall within frequency bands causing high noiselevels, to the carriers. The discrete multitone (DMT) technique is oneof the techniques for transmitting data using a plurality of carriers,and is adopted as a modulating technique to be adapted to the asymmetricdigital subscriber line (ADSL) technique.

[0012]FIG. 24 is an explanatory diagram concerning a data transmissiondevice proposed previously. The data transmission device is equivalentto the modem that is included in the aforesaid power line carriercommunication system and connected to the house wiring in order totransmit or receive data. Referring to FIG. 24, there is shown a codeconverter 111 having a scrambling (SCR) capability, a series-to-parallel(S/P) conversion capability, a Gray code-to-natural binary (G/N)conversion capability, and a finite sum arithmetic capability.

[0013] There are also shown a signal element generation unit 112, aninverse fast Fourier transform (IFFT) unit 113 having a guard time GTaddition capability, a zero element insertion unit 114, a rolloff filter(ROF) 115, a modulator (MOD) 116, a digital-to-analog (D/A) converter117, a low-pass filter (LPF) 118, a transmission clock generation unit(TX-CLK) 119, a transmission line (TX-line), and a reception line(RX-line). Moreover, there are shown a bandpass filter (BPF) 120, ananalog-to-digital (AID) converter 121, a demodulator (DEM) 122, arolloff filter (ROF) 123, a reception clock distribution unit (RX-CLK),a timing sample unit (TIM) 125. Furthermore, there are shown a phaselocked loop (PLL) 126 including a voltage-controlled crystal oscillator(VCXO), a noise removing unit 127, a fast Fourier transform (FFT) unit128 having a guard time (GT) deletion capability, a signalidentification unit (DEC) 129, and a code converter 130 having adifference arithmetic capability, a natural binary-to-Gray code (N/G)conversion capability, a parallel-to-series (P/S) conversion capability,and a de-scrambling (DSCR) capability. Moreover, a transmission signalSD and a reception signal RD are also shown.

[0014] A clock pulse generated by the transmission clock generation unit119 is applied to the circuit elements. To the zero element insertionunit 114, the clock pulse is applied as a timing signal that determinesthe timing of inserting a zero element. The code converter 111 scramblesthe transmission signal SD, and converts it into the same number offrequency components as the number of carriers so that the frequencycomponents will be transmitted in parallel with one another. Moreover,the code converter 111 converts the number system adopted for thetransmission signal SD converted from the Gray code into the naturalbinary. Moreover, the code converter 111 performs finite sum arithmeticso that a receiving side can perform difference arithmetic. Thereafter,the signal element generation unit 112 gives a Nyquist interval to thesignal elements. The inverse FFT unit 113 adds a guard time GT to theresultant signal and performs inverse FFT on the signal. The zeroelement insertion unit 114 inserts zero elements having a zero levelaccording to the timing signal that determines the timing of insertingzero elements. The rolloff filter 115 re-shapes the waveform of thesignal. The modulator 116 digitally modulates the signal. The D/Aconverter 117 converts the signal into an analog form. The low-passfilter 118 places low-frequency components, which fall within atransmission band ranging, for example, 10 kHz to 450 kHz, on thetransmission line TX-line. In this case, the transmission line TX-lineand reception line RX-line are linked with the house wiring and couplingfilters between them.

[0015] The reception clock distribution unit 124 distributes a clockpulse, which is produced based on a clock pulse sent from the phaselocked loop 126, to the circuit elements. A signal received over thereception line RX-line has the frequency components thereof, which fallwithin, for example, a range from 10 kHz to 450 kHz, passed through thebandpass filter 120. The A/D converter 121 digitizes the signal. Thedemodulator 122 demodulates the signal. The rolloff filter 123 re-shapesthe waveform of the signal. The noise removing unit 127 detects thenoise levels of noises superposed on zero elements according to theclock pulse sent from the reception clock distribution unit 124,interpolates the noise levels to calculate the noise level of a noisesuperposed on a signal element, and removes the noise from the signalelement. The FFT unit 128 deletes the guard time GT, and transforms thesignal from the time domain into the frequency domain. The signalelement identification unit 129 identifies the signal. The codeconverter 130 performs parallel-to-series conversion, de-scrambling,difference arithmetic, and natural binary-to-Gray code conversion so asto produce the reception signal RD.

[0016]FIGS. 25A to 25D are explanatory diagrams concerning a zeroelement inserted by the zero element insertion unit 114 shown in FIG.24, and noise removal. Referring to FIGS. 25A to 25D, the transmissionspeed for signal elements S (25A) shall be 192 kB, and zero elementsindicated with black dots are inserted into a transmission signal (25B).By copying one bit, one zero element can be equivalently insertedbetween signal elements S. The insertion of zero elements doubles thetransmission speed to 384 kB. A received signal (25C) has a noise Nsuperposed on the signal elements S and zero elements respectivelyduring transmission. The noise N superposed on the zero elements issampled. As the same noise as the sampled noise N is superposed on thesignal elements, the noise N is removed from the signal elements.Consequently, a reception signal or a signal resulting from noiseremoval (25D) can be restored.

[0017] Incidentally, the insertion of zero elements is such that onezero element may be inserted among a plurality of signal elements or aplurality of zero elements may be inserted between signal elements. Forexample, when two zero elements are inserted between signal elements, ifraw data falls within a frequency band of 128 kHz wide, the frequencyband is expanded to be 384 kHz wide.

[0018] Generally, an error correcting means for data appends an errorcorrecting code to data, and detects, based on the error correctingcode, if an error has occurred in the data. If an error has occurred,the error is corrected. However, since the error correcting code iscomposed of a plurality of bits, the appending of the error correctingcode may lead to the deterioration of the efficiency in high-speedtransmission. This poses a problem.

[0019] Moreover, as mentioned above, when the means for inserting zerocomponents, sampling a noise superposed on the zero components, andcanceling a noise superposed on signal components according to thesampled noise is adapted in the power line carrier communication system,high-speed transmission can be achieved with the adverse effect ofnoises minimized. However, the distribution of noise is, as shown inFIG. 23B, observed over a plurality of frequency bands, and the noiselevels thereof are relatively high. Moreover, the noise levels andfrequency bands often vary time-sequentially. Consequently, the noisecomponents cannot be removed reliably. This leads to occurrence oferrors in identifying data.

[0020] When multilevel modulation is adopted, the modulated signalelements of a received signal vary greatly due to the adverse effect ofnoises. An error in identifying data occurs frequently. This poses aproblem in that it is hard to increase the number of signal levels to bemodulated through multilevel modulation that enables high-speedtransmission.

SUMMARY OF THE INVENTION

[0021] Accordingly, an object of the present invention is to effectivelycorrect errors by transmitting data at a high speed with a plurality ofchannels assigned to the data, and using at least one channel for errorcorrection.

[0022] An error correction device in accordance with the presentinvention will be described with reference to FIG. 3. The errorcorrection device corrects errors in data items sent on a plurality ofchannels. At least one of the plurality of channels is used as an errorcorrection channel. An identification unit 29 calculates the exclusiveOR of data items sent on the other channels, adopts the calculatedexclusive OR as data sent on the error correction channel, and thusidentifies the data items sent on the plurality of channels. Anexclusive OR unit 37 calculates the exclusive OR of the identified dataitems sent on the plurality of channels. A signal quality detection unit38 detects the signal qualities concerning the plurality of channels. Ifan output of the exclusive OR unit 37 is a predetermined specific valuesuch as 0, an identifying and processing unit 39 provides the identifieddata items sent on the channels other than the error correction channelas they are. If the output of the exclusive OR unit 37 is not thepredetermined specific value such as 0, the identifying and processingunit 39 replaces the identified data item, which is sent on the channelwhose signal quality is detected to be the worst by the signal qualitydetection unit 38, with the exclusive OR of the identified data itemssent on the channels other than the channel whose signal quality is theworst. The identifying and processing unit 39 then provides theresultant data. An error correction device may include theidentification unit 29, exclusive OR unit 37, signal quality detectionunit 38, and identifying and processing unit 39.

[0023] Otherwise, channels are associated with the sides of each surfaceof a regular polyhedron. A channel associated with one side of eachsurface of the regular polyhedron is used as an error correctionchannel. An identification unit 29 calculates the exclusive OR of dataitems sent on channels associated with the remaining sides of eachsurface, adopts the calculated exclusive OR as data sent on the errorcorrection channel, and thus identifies the data items sent on theplurality of channels. An exclusive OR unit 37 calculates the exclusiveOR of identified data items sent on the error correction channel and theother channels associated with the same surface as the error correctionchannel is. A signal quality detection unit 38 detects the signalqualities concerning the plurality of channels that is associated withthe sides of the surface and grouped together. If an output of theexclusive OR unit that calculates the exclusive OR of data items sent onchannels associated with a surface is a predetermined specific valuesuch as 0, an identifying and processing unit 39 provides the identifieddata items, which is sent on the channels other than the errorcorrection channel associated with the same surface as the channels are,as they are. If the output of the exclusive OR unit is not thepredetermined specific value such as 0, the identifying and processingunit 39 replaces the identified data, which is sent on the channel whosesignal quality is detected to be the worst by the signal qualitydetection unit 38, with the exclusive OR of the identified data itemssent on the channels other than the channel whose signal quality is theworst. The identifying and processing unit 39 then provides theresultant data. An error correction device in accordance with thepresent invention may include the exclusive OR unit 37, signal qualitydetection unit 38, and identifying and processing unit 39.

[0024] Otherwise, channels are associated with the sides of each surfaceof a regular polyhedron. Channels associated with the sides of a surfaceof the regular polyhedron and with the sides of a plurality of surfacesadjoining the surface are grouped together. If the exclusive OR ofidentified data items sent on the channels associated with a surface isnot a predetermined specific value such as 0, an identifying andprocessing unit regards the same number of channels as the number ofsurfaces of the regular polyhedron, of which signal qualities aredetected to have deteriorated by the signal quality detection unit, aserror channels. The identifying and processing unit then solvessimultaneous equations whose unknowns are data items assigned the errorchannels so as to thus correct the data items sent on the errorchannels. An error correction device in accordance with the presentinvention may include the identifying and processing unit.

[0025] Otherwise, each of a plurality of channels is assigned to aplurality of bits. One bit or a plurality of bits out of the pluralityof bits is manipulated as a subset. The plurality of channels is groupedin units of a predetermined number of channels. One channel of eachgroup is used as an error correction channel. An exclusive OR unitcalculates the exclusive OR of subsets sent on the channels other thanthe error correction channel, adopts the calculated exclusive OR as asubset sent on the error correction channel, and thus identifies thedata items sent on the plurality of channels. The exclusive OR unit thencalculates the exclusive OR of the identified data items of the subsetssent on the channels belonging to a group. A signal quality detectionunit detects the signal qualities concerning channels belonging to thegroup according to the subsets. If an output of the exclusive OR unit isa predetermined specific value such as 0, an error correction unitprovides the identified data items of the subsets, which are sent on thechannels other than the error correction channel, as they are. If theoutput of the exclusive OR unit is not the predetermined value such as0, the error correction unit replaces the identified data, which is senton the channel whose signal quality is detected to be the worst by thesignal quality detection unit, with the exclusive OR of the identifieddata items of the subsets sent on the channels other than the channelwhose signal quality is the worst. The error correction unit thenprovides the resultant data. An identification unit identifies bitsother than the subsets, and provides the identified bits together withthe subsets whose errors have been corrected by the error correctionunit. An error correction device in accordance with the presentinvention may include the exclusive OR unit, a signal quality detectionunit, an error correction unit, and an identification unit.

[0026] Next, an error correction method in accordance with the presentinvention will be described below. Herein, at least one channel out of aplurality of channels is used as an error correction channel. At anidentification step, the exclusive OR of data items sent on theremaining channels other than the error correction channel is calculatedand adopted as data sent on the error correction channel. Thus, the dataitems sent on the plurality of channels are identified. At a detectionstep, a signal quality detection unit or the like detects the signalqualities concerning channels according to the identified data items. Atan exclusive-OR step, an exclusive OR unit calculates the exclusive ORof the identified data items sent on the plurality of channels. At anoutput step, if the calculated exclusive OR is a predetermined specificvalue such as 0, the identified data items sent on the channels otherthan the error correction channel are provided as they are. If theexclusive OR is not the predetermined specific value such as 0, the datasent on the channel whose signal quality is detected to be the worst asa result of detecting the signal qualities concerning the plurality ofchannels is replaced with the exclusive OR of the identified data itemssent on the channels other than the channel. The resultant data is thenprovided. An error correction method according to the present inventionmay include the identification step, a detection step, an exclusive-ORstep, and an output step.

[0027] Otherwise, channels are associated with the sides of each surfaceof a regular polyhedron, and a channel associated with one side of eachsurface of the regular polyhedron is used as an error correctionchannel. At an identification step, the exclusive OR of data items senton channels associated with the remaining sides of each surface iscalculated and adopted as data sent on the error correction channel.Thus, the data items sent on the plurality of channels are identified.At a detection step, the signal qualities concerning channels aredetected based on the identified data items. At an output step, if theexclusive OR of identified data items sent on the channels associatedwith a surface is a predetermined specific value such as 0, theidentified data items sent on the channels other than the errorcorrection channel associated with the same surface as the channels areprovided as they are. If the exclusive OR is not the predeterminedspecific value such as 0, the identified data sent on the channel whosesignal quality is detected to be the worst as a result of detecting thesignal qualities concerning the channels is replaced with the exclusiveOR of the identified data items sent on the other channels other thanthe channel. Then, the error-corrected data is provided. An errorcorrection method in accordance with the present invention may includethe identification step, a detection step, and an output step.

[0028] Otherwise, channels are associated with the sides of each surfaceof a regular polyhedron. Channels associated with the sides of eachsurface of the regular polyhedron and with the sides of a plurality ofsurfaces adjoining the surface are grouped together. At a correctionstep, if the exclusive OR of identified data sent on channels associatedwith a surface is not a predetermined specific value such as 0, the samenumber of channels as the number of surfaces of the regular polyhedron,of which signal qualities are found to have deteriorated as a result ofdetecting the signal qualities concerning the channels, are regarded aserror channels. Simultaneous equations whose unknowns are data itemsassigned to the error channels are solved in order to thus correct thedata items sent on the error channels. Otherwise, if the exclusive OR ofidentified data items sent on the channels associated with a surface ofthe regular polyhedron is not the predetermined specific value such as0, the same number of channels as the number of surfaces of the regularpolyhedron, of which signal qualities are found to have deteriorated asa result of detecting the signal qualities concerning channels, areregarded as error channels. A coefficient assigned to the error channelassociated with a surface is set to 1. The other error channelassociated surfaces are successively selected, and the coefficientsassigned to the channels are set to 0. Finally, coefficients assumingany values other than 0 are detected, and channels assigned thecoefficients are regarded as channels on which an error has occurred.The errors are then corrected. An error correction method in accordancewith the present invention may include the correcting step.

[0029] Otherwise, a signal to which each of a plurality of channels isassigned is composed of a plurality of bits. One bit or a plurality ofbits out of the plurality of bits is manipulated as a subset. At leastone of the plurality of channels is used as an error correction channel.At an exclusive OR step, the exclusive OR of subsets sent on the otherchannels is calculated and adopted as a subset sent on the errorcorrection channel. Thus, the data items sent on the plurality ofchannels are identified. The exclusive OR of the data items of thesubsets is calculated. At a detection step, the signal qualitiesconcerning the channels are detected based on the subsets. At an outputstep, if the calculated exclusive OR is a predetermined specific valuesuch as 0, the identified data items of the subsets sent on the channelsother than the error correction channel are provided as they are. If thecalculated exclusive OR is not the predetermined specific value such as0, the identified data sent on the channel whose signal quality is foundto be the worst as a result of detecting the signal qualities concerningchannels is replaced with the calculated exclusive OR of the identifieddata items of the subsets sent on the channels other than the channelwhose signal quality is the worst. The resultant data is then provided.At an identification step, bits other than the subsets are identifiedand provided together with the subsets. Otherwise, a plurality ofchannels is divided into a plurality of groups, and one channel out ofeach group is used as an error correction channel. At an errorcorrection step, the exclusive OR of subsets sent on the channels otherthan the error correction channel belonging to each group is calculatedand adopted as a subset sent on the error correction channel. An errorin the subset sent on the channel whose signal quality is found to bethe worst as a result of detecting the signal qualities concerning thechannels of each group is corrected based on the identified data itemsof the subset. An error correction method in accordance with the presentinvention may include the exclusive OR step, a detection step, an outputstep, an identification step, and an error correction step.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is an explanatory diagram concerning an embodiment of thepresent invention;

[0031]FIG. 2 is an explanatory diagram concerning an error correctionand data production unit employed in the embodiment of the presentinvention;

[0032]FIG. 3 is an explanatory diagram concerning an error correctionunit employed in the embodiment of the present invention;

[0033]FIGS. 4A to 4E are explanatory diagrams concerning a regularpolyhedron;

[0034]FIG. 5 is an explanatory diagram concerning the relationshipbetween a regular dodecahedron and channels;

[0035]FIG. 6 is an explanatory diagram concerning the relationshipbetween surface numbers assigned to the surfaces of a regulardodecahedron and channels which is employed in the embodiment of thepresent invention;

[0036]FIGS. 7A to 7B are explanatory diagrams concerning grouping ofeach surface and surrounding surfaces which is employed in theembodiment of the present invention;

[0037]FIG. 8 is an explanatory diagram concerning assignment ofcoefficient values;

[0038]FIG. 9 is an explanatory diagram concerning simultaneousoccurrence of errors on twelve channels;

[0039]FIGS. 10A to 10B are explanatory diagrams concerning channels onwhich an error has occurred and the first step;

[0040]FIGS. 11A to 11B are explanatory diagrams concerning the secondstep and the third step;

[0041]FIGS. 12A to 12B are explanatory diagrams concerning the fourthstep and the fifth step;

[0042]FIGS. 13A to 13B are explanatory diagrams concerning the sixthstep and the seventh step;

[0043]FIGS. 14A to 14B are explanatory diagrams concerning the eighthstep and the ninth step;

[0044]FIGS. 15A to 15B are explanatory diagrams concerning the tenthstep and the eleventh step;

[0045]FIG. 16 is an explanatory diagram concerning the twelfth step;

[0046]FIGS. 17A to 17B are explanatory diagrams concerning signalelements;

[0047]FIG. 18 is an explanatory diagram concerning error correctionperformed on subsets according to an embodiment of the presentinvention;

[0048]FIG. 19 is an explanatory diagram concerning error correctionperformed on subsets according to the embodiment of the presentinvention;

[0049]FIGS. 20A to 20D are explanatory diagrams showing eye patternviews and spectra;

[0050]FIGS. 21E to 21H are explanatory diagrams showing eye patternviews and spectra;

[0051]FIG. 22 is an explanatory diagram concerning a power line carriercommunication system;

[0052]FIGS. 23A to 23B are explanatory diagrams concerning noisecanceling;

[0053]FIG. 24 is an explanatory diagram concerning a previously proposeddata transmission device; and

[0054]FIGS. 25A to 25D are explanatory diagrams concerning zero elementinsertion and noise removal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0055]FIG. 1 is an explanatory diagram of an embodiment of the presentinvention. The present invention is adapted to the data transmissiondevice included in the power line carrier communication system shown inFIG. 22. There are shown an error correction and data production unit 10and a code converter 11 having a scrambling (SCR) capability, aseries-to-parallel (S/P) conversion capability, a Gray code-to-naturalbinary (G/N) conversion capability, and a finite sum arithmeticcapability. Moreover, there are shown a signal element generation unit12, an inverse fast Fourier transform (IFFT) unit 13 having a guard timeGT addition capability, a zero element insertion unit 14, a rollofffilter (ROF) 15, a modulator (MOD) 16, a D/A converter (D/A) 17, alow-pass filter (LPF) 18, and a transmission clock generation unit(TX-CLK) 19. A transmission line TX-line and a reception line RX-lineare also shown.

[0056] Moreover, there are shown a bandpass filter (BPF) 20, an A/Dconverter (A/D) 21, a demodulator (DEM) 22, a rolloff filter (ROF) 23, areception clock distribution unit (RX-CLK) 24, a timing sample unit(TIM) 25, a noise removing unit 27, and a fast Fourier transform (FFT)unit 28 having a guard time GT deletion capability. Moreover, there area signal identification unit (DEC) 29 and a code converter 30 having adifference arithmetic capability, a natural binary-to-Gray code (N/G)conversion capability, a parallel-to-series (P/S) conversion capability,and a de-scrambling (DSCR) capability. Moreover, there are shown anerror correction unit 31, a signal quality detection unit 32, atransmission signal SD, and a reception signal RD.

[0057] According to the present embodiment, the error correction anddata production unit 10, error correction unit 31, and signal qualitydetection unit 32 are added to the data transmission device shown inFIG. 24. The inclusion of the error correction unit 31 and signalquality detection unit 32 realizes the capability of an error correctiondevice. The iterative description of the circuit elements identical tothose of the data transmission device shown in FIGS. 20A to 20D will beomitted. Moreover, the error correction and data production unit 10 usesone of, for example, 21 channels as an error correction channel, andcalculates the exclusive OR (XOR) or modulo sum of transmission dataitems to which the remaining twenty channels are assigned. The errorcorrection and data production unit 10 then transfers the calculatedexclusive OR or modulo sum or the reverse thereof as transmission data,to which the error correction channel is assigned, to the signal elementgeneration unit 12. Signal elements used for multilevel modulation areassigned to the data items of the transmission data.

[0058] A received signal is transferred to the signal quality detectionunit 32 together with the results of identification performed by thesignal identification unit 29. The signal qualities concerning thechannels are then detected. The error correction unit 31 calculates theexclusive OR (XOR) or modulo sum of data items identified by the signalidentification unit 29 and sent on the channels. The signal qualitydetection unit 32 converts errors, that is, differences between thesignal elements of the received signal and the results of identificationinto scalars. A difference of the result of integration of the scalarsfrom a reference value predefined in relation to an error ratio iscalculated. Consequently, a binary quality evaluation signal isproduced. (For details, refer to, for example, Japanese Examined PatentApplication Publication No. 58-54686.) According to the presentembodiment of the present invention, the binary signal is not producedbut the result of integration or the difference from the reference valueis adopted as a signal quality value as it is.

[0059] The exclusive OR or modulo sum of data items identified by theerror correction unit 31 or the exclusive OR or modulo sum of data itemssent on the channels other than the error correction channel from thetransmitting side is calculated and adopted as transmission data sent onthe error correction channel. Otherwise, the transmitting side reversesthe calculated exclusive OR or modulo sum, and adopts the resultant dataas transmission data sent over the error correction channel. In thiscase, if no error occurs, the transmission data is 1 that is apredetermined specific value. The identified data items sent on the 20channels are transferred to the code converter 30 as they are. If theexclusive OR is not the specific value but 0, it means that an error hasoccurred in data sent on any channel. The data sent on the channel whosesignal quality is detected to be the worst by the signal qualitydetection unit 32 is replaced with the exclusive OR or modulo sum ofdata items sent on the channels other the channel. Consequently, thedata containing an error is corrected.

[0060]FIG. 2 is an explanatory diagram concerning the error correctionand data production unit employed in the present embodiment of thepresent invention. An exclusive OR unit 35 calculates the exclusive ORof data items to which the channels are sequentially assigned.Alternatively, the exclusive OR unit 35 may be realized with logicalgates that calculate the exclusive OR of data items in parallel with oneanother or with software that calculates the exclusive OR. The exclusiveOR is equivalent to the modulo sum of data items to which the channelsare assigned. The exclusive OR unit employed in the present inventionincludes a module adder. The exclusive OR unit 35 calculates theexclusive OR of data items assigned channels from channel 0 to channel19, and adopts the calculated exclusive OR as transmission data assignedthe error correction channel. The exclusive OR of the transmission dataassigned the error correction channel and the transmission data itemsassigned channels 0 to 19 is 0.

[0061] For example, assuming that channels 0 and 1 are used as channelsassigned to data items and channel 2 is used for error correction, theexclusive OR operation will be described briefly. The exclusive ORoperation is an operation whose result is 1 if two bits are differentfrom each other and whose result is 0 if two bits are identical to eachother. The exclusive OR operation is therefore identical to moduleaddition. Assuming that data assigned channel 0 is 01 and data assignedchannel 1 is 11, the exclusive OR of the data items is 10. The data of10 is adopted as data assigned the error correction channel. Theexclusive OR of data items assigned channels 0 and 1, and the errorcorrection channel that is channel 2; that is, the exclusive OR (ormodulo sum) of 01, 11, and 10 is 00. The data assigned the errorcorrection channel and calculated by the exclusive OR unit 35, and thedata items assigned channels 0 to 19 are transferred to the signalelement generation unit 12, adopted as modulating signal elements, andtransferred to the inverse FFT unit 13 shown in FIG. 1.

[0062]FIG. 3 is an explanatory diagram concerning the error correctionunit employed in the embodiment of the present invention. Data itemssent on channels 0 to 19 and identified by the signal identificationunit (DEC) 19 are transferred to the identifying and processing unit 39and exclusive OR unit (XOR) 37 respectively. Identified data sent on theerror correction channel is transferred to the exclusive OR unit 37. Theexclusive OR unit 37 has the same circuitry as the aforesaid exclusiveOR unit 35. The signal identification unit 29 transfers the vectors ofthe differences between the signal elements of a received signal sent onchannels 0 to 19 and the error correction channel and the identifieddata items to the signal quality detection unit (SQD) 38. The signalidentification unit 29 also transfers error signals, each of whichincludes a phase error component and an amplitude error component ofeach signal element, to the signal quality detection unit (SQD) 38.Specifically, the exclusive OR unit 37 calculates the exclusive OR ofthe identified data items sent on channels 0 to 19 and the identifieddata sent on the error correction channel, and transfers the calculatedexclusive OR to the identifying and processing unit 39. The signalquality detection unit 38 having the same capability as the signalquality detection unit 32 shown in FIG. 1 detects the signal qualitiesconcerning the channels, and transfers them to the identifying andprocessing unit 39.

[0063] Assume that the transmitting side adopts the exclusive OR of dataitems assigned channels 0 to 19 as the transmission data assigned theerror correction channel. In this case, if the exclusive OR of theidentified data items sent on the channels which is calculated by theexclusive OR unit 37 is not 0 (XOR is not equal to 0), the identifyingand processing unit 39 replaces the data, which is sent on the channelwhose signal quality is detected to be the worst by the signal qualitydetection unit 38 as a result of detecting the signal qualitiesconcerning the channels, with the exclusive OR of the identified dataitems sent on the channels other than the channel. The resultant signalis regarded as received data. If the exclusive OR is 0 (XOR=0), theidentified data sent on the channels are provided as they are, that is,the identified data items sent on channels 0 to 19 are adopted asreceived data items as they are.

[0064] For example, assuming that the exclusive OR is not 0, if thechannel whose signal quality is detected to be the worst is channel 2,the identifying and processing unit 39 replaces the exclusive OR of thedata items sent on channels 0 and 1, and channels 3 to 19 and errorcorrection channel other than channel 2 with the data sent on channel 2and adopts the resultant data as received data. The employment of theerror correction channel makes it possible to correct data sent on oneof a plurality of channels.

[0065] According to the foregoing embodiment, one of a plurality of datachannels is used as an error correction channel. Namely, an errorcontained in data sent on any one of a plurality of data channels iscorrected. A plurality of channels out of a plurality of data channelsmay be used as error correction channels. The relationships between thedata channels and error correction channels may be discussed using aregular polyhedron. Thus, errors occurring on a plurality of datachannels may be corrected. Moreover, data assigned each channel may benot only realized with a single bit but also composed of a plurality ofbits. For example, data items assigned channels 0 to 19 may each becomposed of two bits. If the modulo sum of the data items is 01,transmission data assigned an error correction channel is set to 11 sothat the modulo sum of 01 and 11 will be 00. The transmission dataassigned the error correction channel may be fixed to a predeterminedspecific value according to the calculated exclusive OR or modulo sum.

[0066] The regular polyhedron may be, as shown in FIGS. 4A to 4E, aregular tetrahedron, a regular hexahedron, a regular octahedron, aregular dodecahedron, or a regular icosahedron. For example, the regulardodecahedron has twelve regular pentagonal surfaces and thirty sides.When channels are associated with the sides, thirty channels areassociated with the sides of the twelve surfaces. In this case, one offive channels associated with each surface is used as an error channel.Likewise, in the case of the regular tetrahedron, four out of all thesix channels are used as error correction channels. In the case of theregular hexahedron, six out of all the twelve channels are used as errorcorrection channels. In the case of the regular octahedron, eight out ofall the twelve channels are used as error correction channels. In thecase of the regular dodecahedron, twelve out of all the thirty channelsare used as error correction channels. In the case of the regularicosahedron, twenty out of all the thirty channels are used as errorcorrection channels.

[0067]FIG. 5 is an explanatory diagram concerning the relationshipbetween a regular dodecahedron and channels. Side numbers 0 to 29 areassigned to the sides of the regular dodecahedron. Surface 1 has fivesides of side numbers 0 to 4. Channels 0 to 4 are associated with thesides. The exclusive OR of transmission data items assigned the fivechannels is calculated. One of the channels associated with the sides isused as an error correction channel, and the calculated exclusive OR isadopted as transmission data assigned the error correction channel.Thus, an error occurring in data to which one of the channels associatedwith surface 1 is assigned can be corrected.

[0068] Likewise, channels are associated with the sides of each surfaceother than surface 1, and the exclusive OR of transmission data itemsassigned the five channels is calculated. One of the channels is used asan error correction channel, and the calculated exclusive OR is adoptedas transmission data assigned the error correction channel. Any ofchannel numbers 0 to 29 grouped and associated with any of surfacenumbers 1 to 12 are indicated with round marks in FIG. 6. When one ofthe channels associated with a surface is used as an error correctionchannel, twelve error correction channels are available in total.

[0069] For example, assuming that the arrangements shown in FIG. 2 andFIG. 3 and employed in the embodiment are designed to manipulate fivechannels, when twelve pairs of the arrangements are included, thirtychannels can be associated with the sides of a regular dodecahedron anderrors occurring in data items assigned twelve channels out of thethirty channels can be corrected. In other words, the embodiment shownin FIG. 2 and FIG. 3 may be discussed in such a manner that: channelsare associated with the sides of a regular twenty-one-sided polygon on atwo-dimensional plane; one of the channels is used as an errorcorrection channel; and the other channels associated with the othertwenty sides are used as data channels (channels 0 to 19). Consequently,when channels are associated with the sides of each surface of a regularpolyhedron, an error correction device includes the error correctionunit shown in FIG. 3 and designed to manipulate channels associated withthe sides of each surface thereof.

[0070] When the relationships of the sides of the surfaces of a regulardodecahedron to the surfaces thereof is adopted, twelve out of thirtychannels are used as error correction channels. An error contained inidentified data sent on one channel whose signal quality is the worstamong the channels associated with the same surface can be corrected.Consequently, errors occurring on twelve channels, that is, the samenumber of channels as the number of error correction channels can becorrected.

[0071] According to the embodiment, as mentioned above, an erroroccurring on one channel among the channels associated with each surfacecan be corrected. However, errors occurring on a plurality of channelsamong the channels associated with each surface cannot be corrected.Therefore, twelve groups of (20−12=8) eight or more channels are definedin order to enable correction of errors occurring on any twelve channelsamong the twenty channels. Twelve simultaneous equations are solved.Thus, errors occurring on up to twelve channels among all the channelsassociated with all the surfaces can be corrected. As for grouping ofchannels, for example, channels associated with the sides of one surfaceand channels associated with the sides of surfaces adjoining the surfaceare grouped together.

[0072] Coefficients are assigned at random to channels so that thetwelve simultaneous equations will be established mutuallyindependently. A modulo sum is calculated and transmitted on an errorcorrection channel. As shown in FIG. 7A, channel numbers associated withsurface numbers and grouped together are indicated with round marks. Inthis case, when error correction channels EC each defined as one of thechannels associated with each surface are, as shown in FIG. 7B, channels20 to 31, any of a total of 32 channels can be grouped together.Coefficients to be assigned to the channels assume, as shown in FIG. 8,for example, 1, 2, or 3. Channels 0 to 19 associated with surfaces 1 to12 are used as data channels, and channels 20 to 31 are used as errorcorrection channels EC. The modulo sum of data items assigned the datachannels associated with a surface is adopted as transmission dataassigned an error correction channel EC. Moreover, the coefficientvalues are not limited to 1, 2, and 3. A larger number of coefficientvalues may be adopted and assigned to channels at random in advance.

[0073] For example, assume that data items assigned channels 0 to 19associated with surface 1 are data items D0 to D19. The data items aremultiplied by the coefficients, and modulo-added, whereby data assignedthe error correction channel EC1 associated with surface 1 iscalculated. The data assigned the error correction channel EC1 shouldmeet the condition expressed as follows: 1×D0+1×D1+3×D3+1×D4+ . . .+2×D17+2×D18+3×D19+EC1=0. Alternatively, the data assigned the errorcorrection channel may be determined so that the modulo sum of the dataand the modulo sum of the products of the data items D0 to D19 by thecoefficients will be any predetermined specific value other than 0. Forexample, assuming that data assigned each channel is composed of twobits, if the modulo sum of the products of the data items DO to D19 bythe coefficients is 01, the data assigned the error correction channelis 11. Namely, since the modulo sum of 01 and 11 is 00, 11 is determinedas the data assigned the error correction channel.

[0074] The receiving side manipulates identified data items representedby signal elements sent on the channels in consideration of the groupingof channels associated with surfaces shown in FIG. 8. The data itemssent on the channels are weighted with the coefficients employed at thetransmitting side, and modulo-added. Twelve simultaneous equations areestablished. If occurrence of an error is detected, twelve channelswhose signal qualities are detected to be the worst to the twelfth worstby the signal quality detection unit are selected. The data itemsassigned the twelve channels are regarded as unknowns. Since the dataitems sent on the other channels contain no error, the data items areregarded as known quantities (constants). The twelve simultaneousequations having the unknowns that are the data items assigned thetwelve channels can be established.

[0075] The twelve simultaneous equations are solved, whereby the errorsoccurring on the twelve channels can be corrected. The twelvesimultaneous equations can be solved using, for example, theGauss-Jordan elimination. For example, the same coefficients as thoseadopted at the transmitting side are, as shown in FIG. 8, assigned tothe channels that are associated with surfaces and grouped together.Assume that a total of 12 channels, that is, channels 0 to 2, channels 8to 10, channels 16 to 18, and channels 24 to 26 which are associatedwith surfaces 1 to 12 are selected as channels whose signal qualitiesare detected to be the worst to the twelfth worst during errordetection. As mentioned above, data items sent on the channels on whichno error has occurred are regarded as known quantities (constants). Forexample, data items sent on the channels associated with surface 1 aremultiplied by the aforesaid coefficients, and modulo-added. Thecalculated modulo sum is adopted as the data item A sent on the errorcorrection channel. The modulo sum of the calculated modulo sum and thedata item A should be 0. This is expressed as an equation of

1×D0+1×D1+3×D2+2×D8+3×D9+3×D10+2×D16+2×D17+2×D18+A=0.

[0076]FIG. 10A shows coefficient values by which data items sent onchannels on which an error has occurred are multiplied.

[0077] Twelve equations like the above one each of which includes amultiplication of identified data items sent on channels associated witheach of surfaces 1 to 12 by the coefficients are created. Referring toFIGS. 10A to 10B to FIG. 16, a procedure of solving the twelve equationswill be described below. FIG. 10B is concerned with the first step ofthe procedure. At the first step, coefficients assigned to channels 0other than channel 0 associated with surface 1 are reset to 0. Thecoefficients assigned to the error channels associated with surface 1assume the values of 1, 1, 3, 2, 3, 3, 2, 2, and 2 respectively.Moreover, the coefficients assigned to channels 0 associated with allthe surfaces have assumed the values of 1, 3, 2, 2, 2, 1, 1, and 3respectively. As the coefficients assigned to channels 0 other thanchannel 0 associated with surface 1 are set to 0, the coefficientassigned to each of channels 1 to 18 associated surface 1 is multipliedby the coefficient originally assigned to each of channels 0 associatedwith surfaces 2 to 12. The product is then subtracted from thecoefficient originally assigned to each channel associated with each ofsurfaces 2 to 12. The resultant coefficient values are manipulated atsubsequent steps.

[0078] For example, the coefficient assigned to channel 0 associatedwith surface 2 assumes a value of 3. Therefore, the coefficientsassigned to channel 1 and the other channels associated with surface 1,that is, 1, 3, 2, 3, 3, 2, 2, and 2 are multiplied by the coefficient 3.The products are then subtracted from the coefficients assigned to thechannels associated with surface 2. Therefore, the coefficients assignedto the channels associated with surface 2 now assume the values of 0,−1, −6, −5, −9, −9, −5, −5, −4, 0, 0, and 0 respectively. Specifically,the coefficient assigned to channel 2 associated with surface 2originally assumes a value 3, and the coefficient assigned to channel 2associated with surface 1 assumes 3. Therefore, 3−3×3=−6. As for channel24 to channel 26, the coefficients assigned to the channels associatedwith surface 1 assume a value of 0. Therefore, 0−3×0=0. Likewise, sincethe coefficient originally assigned to channel 0 associated with surface3 is 2, the coefficients originally assigned to the channels associatedwith surface 3, that is, 2, 1, 2, 1, 3, 2, 0, 1, 1, 0, 0, and 0respectively as shown in FIG. 10A are reset to 0, −1, −4, −3, −4 −4, −3,−3, 0, 0, and 0 respectively as shown in FIG. 10B. Moreover, thecoefficient originally assigned to the channels associated with surface4 are reset to 0, 0, −4, −2, −5, −4, −4, −4, −4, 0, 0, and 0respectively. Likewise, the coefficients assigned to the channelsassociated with surfaces 5 to 13 are manipulated. FIG. 10B shows theresults of the first step.

[0079]FIGS. 11A to 11B are explanatory diagrams concerning the secondstep and the third step. At the second step, the coefficients assignedto channels 1 other than the channel 1 associated with surface 11 arereset to 0. Consequently, the coefficients originally assigned to thechannels associated with surface 1 are reset to 1, 0, 0, −1, 0, 0, 3, 1,1, −1, 0, 0, and 0 respectively. The coefficients originally assigned tothe channels associated with surface 2 are reset to 0, 0, −3, −2, −6,−9, −4, −4, −1, 0, 0, and 0 respectively. At the third step, thecoefficients assigned to channels 2 other than channel 2 associated withsurface 10 are reset to 0. Consequently, the coefficients originallyassigned to the channels associated with surface 1 are reset to 1, 0, 0,−1, 0, 3, 1, 1, −1, 0, 0, and 0 respectively. The coefficientsoriginally assigned to the channels associated with surface 2 are resetto 0, 0, 0, 1, −6, −9, 2, 5, 2, 0, 0, and 0 respectively.

[0080]FIGS. 12A to 12B are explanatory diagrams concerning the fourthstep and the fifth step. At the fourth step, the coefficients assignedto channels 8 other than channel 8 associated with surface 2 are resetto 0. Consequently, the coefficients originally assigned to the channelsassociated with surface 1 are reset to 1, 0, 0, 0, −6, −6, 3, 6, 1, 0,0, and 0 respectively. The coefficients originally assigned to thechannels associated with surface 2 are reset to 0, 0, 0, 1, −6, −9, 2,5, 2, 0, 0, and 0 respectively. Herein, 1 succeeding four 0s is thecoefficient newly assigned to channel 8 associated with surface 2. Thecoefficients assigned to the channels associated with the other surfacesare manipulated similarly.

[0081] At the fifth step, the coefficients assigned to channels 9 otherthan channel 9 associated with surface 12 are reset to 0. Consequently,the coefficients originally assigned to the channels associated withsurface 1 are reset to 1, 0, 0, 0, 0, −2, 5, 6. 7, 0, 0, and 0respectively. The coefficients originally assigned to the channelsassociated with surface 2 are reset to 0, 0, 0, 1, 0, −5, 4, 5, 8, 0, 0,and 0 respectively.

[0082]FIGS. 13A to 13B are explanatory diagrams concerning the sixthstep and the seventh step. At the sixth step, the coefficient assignedto channels 10 other than channel 10 associated with surface 3 are resetto 0. At the seventh step, the coefficients assigned to channels 16other than channel 16 associated with surface 5 are reset to 0.Consequently, for example, the coefficients originally assigned to thechannels associated with surface 1 are reset to 8, 0, 0, 0, 0, 0, 0, 24,−76, 15, 0, and 0 respectively. The coefficients originally assigned tothe channels associated with surface 2 are reset to 0, 0, 0, 1, 0, −5,4, 5, 8, 0, 0, and 0 respectively.

[0083]FIGS. 14A to 14B are explanatory diagrams concerning the eighthstep and the ninth step. At the eight step, the coefficients assigned tochannels 17 other than channel 17 associated with surface 9 are reset to0s. Consequently, the coefficient assigned to channel 17 associated withsurface 9 is reset to −2. At the ninth step, the coefficients assignedto channels 18 other than channel 18 associated with surface 6 are resetto 0. Consequently, the coefficient assigned to channel 18 associatedwith surface 6 is reset to −44.

[0084]FIGS. 15A to 15B are explanatory diagrams concerning the tenthstep and the eleventh step. At the tenth step, the coefficients assignedto channels 24 other than channel 24 associated with surface 4 are resetto 0. Consequently, the coefficient assigned to channel 24 associatedwith surface 4 is reset to 5496. At the eleventh step, the coefficientsassigned to channels 25 other than channel 25 associated with surface 8are reset to 0. Consequently, the coefficient original assigned tochannel 25 associated with surface 8 is reset to 5038872. For example,the coefficient assigned to channel 0 associated with surface 1 nowassumes 9.7004E+14 that means 9.7004×10¹⁴. Actually, the coefficientassumes a value of 97004028454502.

[0085]FIG. 16 is an explanatory diagram concerning the twelfth step. Thecoefficients resulting from the eleventh step and assuming any valuesother than 0 as shown in FIGS. 15A to 15B are reset to 1. For example,the coefficient assigned to channel 0 associated with surface 1 assumesthe value of 1. Therefore, the data sent on channel 0 is recognized aserror data and corrected. Likewise, the coefficient assigned to channel8 associated with surface 2 assumes 1, and the coefficient assigned tochannel 10 associated with surface 3 assumes 1. Consequently, errorscontained in the data items sent on the twelve channels whose signalqualities have deteriorated are corrected. Incidentally, the twelvechannels are listed in FIG. 10A. Assume that the solutions of twelvesimultaneous equations are expressed as, for example, 1×D0+β=0, thatdata is composed of two bits, and that β is set to modulo 4. In thiscase, when β equals 00, data D0 sent on channel 0 is 00. When β equals01, D0 is 11. When β equals 10, D0 is 10. When β equals 11, D0 is 01.Thus, an error is corrected.

[0086] At the first to twelfth steps, the results of multiplication andaddition of coefficients are listed as they are. If modulo addition isperformed at each step, the coefficients can be limited to small values.For example, if the maximum value for the coefficients is set to 3,error-corrected data can be set to modulo 3. A means for solving thesimultaneous equations can be easily realized with dedicated hardware oran arithmetic facility such as a digital signal processor (DSP).

[0087] According to the foregoing embodiment, channels associated withthe sides of the surfaces of a regular polyhedron are divided intogroups in order to correct errors occurring on a plurality of datachannels simultaneously. Alternatively, channels associated with thesides of the surfaces of a regular multi-vesicular body that is agraphic in a four-dimensional space equivalent to the regular polyhedronmay be divided into groups. For example, a regular eight-vesicular bodyhas regular hexahedrons as components, and has sixteen vertexes,thirty-two sides, and twenty-four surfaces. A regular 24-vesicular bodyhas regular octahedrons as components, and has 24 vertexes, 96 sides,and 96 surfaces. Similarly to the polyhedrons, channels associated withthe sides of the surfaces of a regular multi-vesicular body are dividedinto groups. Thus, errors may be corrected.

[0088] In a system to which the quadrature amplitude modulation (QAM) orany other multilevel modulation technique is adapted, for example,256-level QAM may be implemented using an 8-bit signal. In this case,the channels are assigned to the eight respective bits, the exclusive ORof the bits is adopted as a bit assigned an error correction channel. Inthis case, data is composed of seven bits. One bit is adopted as anerror correcting bit (assigned the error correction channel). Accordingto the multilevel modulation technique, if the distance between signalelements is so large, an identification error will not occur. Therefore,all bits are not manipulated but part of all bits is manipulated as asubset. Error correction is performed on the subset. The remaining bitsare identified by excluding signal elements that represent the subset.This is equivalent to a case where the distance between signal elementsis extended. Therefore, the occurrence of the identification error canbe minimized.

[0089]FIG. 17A and FIG. 17B are explanatory diagrams concerning signalelements. FIG. 17A shows an example of locations of signal elements thatrepresent five bits. Thus, the signal elements or modulating signalelements express 32 levels. Three bits out of five bits are manipulatedas a subset. Depending on what combination of values such as “000” or“111” the bits assume, the signal elements are located at points shownin any of a to h in FIG. 17B. Two bits of each subset are represented bysignal elements indicated with black dots. Therefore, three out of fivebits are manipulated as a subset. The aforesaid error correction isperformed on the subset. An error in identifying the remaining two bitsis minimized because the distance between signal elements representingthe remaining two bits extends. In other words, part of a plurality ofbits is manipulated as a subset. The exclusive OR of the bits belongingto the subset is calculated and adopted as an error correction bit. Anerror is corrected based on the error correction bit. The remaining bitsare identified as they usually are. In this case, since the distancebetween signal elements representing the remaining bits is extended, anerror in identifying the remaining bits is minimized.

[0090]FIG. 18 is an explanatory diagram concerning error correctionperformed on a subset according to an embodiment of the presentinvention. Bit numbers and channel numbers are listed in FIG. 18. Eachof thirty-two channels of channels 0 to 31 is assigned to a signalcomposed of eight bits of bit number 0 to bit number 7. On each channel,a signal is transmitted according to the 256-level QAM. Moreover, thechannels are grouped in fours. One of four channels belonging to onegroup is used as an error correction channel. The exclusive OR ofsubsets of bits assigned the channels other than the error correctionchannel is calculated and adopted as a subset of bits assigned the errorcorrection channel.

[0091] For example, two bits of bit 0 and bit 1 are manipulated as asubset. The exclusive OR of bits 0 assigned channels 0 to 2 is adoptedas an error correcting bit that serves as bit 0 assigned channel 3 whichis an error correction channel. The exclusive OR of bits 1 assignedchannels 0 to 2 is adopted as an error correction bit of bit 1 assignedchannel 3. Incidentally, D in FIG. 18 denotes a data bit, EC denotes anerror correcting bit. Since the error correcting bits are thus produced,data assigned channel 3, 7, 11, 15, 19, 23, 27, or 31 is composed of 6bits.

[0092] Moreover, the exclusive OR of bits 0 assigned to channels 4 to 6is adopted as an error correcting bit that serves as bit 0 assigned tochannel 7. The exclusive OR of bits 1 assigned channels 4 to 6 isadopted as an error correcting bit that serves as bit 1 assigned channel7. Likewise, error correcting bits are produced for subsets. As far aserror correction of a subset of two bits is concerned, the errorcorrecting means employed in the embodiment described in conjunctionwith FIG. 2 and FIG. 3 is useable. As for the remaining six bits otherthan the subset of two bits, the signal elements other than the signalelements representing the two bits of the subset are identified. This isequivalent to a case where the distance between the signal elements isextended. Consequently, an error in identifying the signal elements isminimized.

[0093]FIG. 19 is an explanatory diagram concerning error correctionperformed on a subset according to an embodiment of the presentinvention. A fast Fourier transform (FFT) unit 50 has a guard time GTdeletion capability and corresponds to the fast Fourier transform unit28 shown in FIG. 1. There are shown an error correction/identificationand processing unit 51, a hard decision unit 52, an exclusive OR unit53, a signal quality detection unit (SQD) 54, an error correction unit55, and a soft decision unit 56. The error correction/identification andprocessing unit 51 has the capabilities of the signal identificationunit 29, error correction unit 31, and signal quality detection unit 32shown in FIG. 1. As shown in FIG. 18, each channel is assigned to asignal composed of eight bits. Two bits out of the eight bits aremanipulated as a subset. The arrangement shown in FIG. 19 manipulatessubsets assigned channels 0 to 3.

[0094] The error correction/identification and processing unit 51transfers an output of the FFT unit 50 into the hard decision unit 52and soft decision unit 56 respectively. The results of harddetermination performed by the hard decision unit 52, that is, twoidentified bits of bit 0 and bit 1 are transferred to the errorcorrection unit 55 and exclusive OR unit 53 respectively. As shown inFIG. 18, the exclusive OR of data items sent on channels 0 to 2 isadopted as transmission data sent on channel 3. If the exclusive OR ofthe results of hard determination which is calculated by the exclusiveOR unit 53 is 0, subsets sent on channels 0 to 2 are judged to containno error. If the exclusive OR is not 0, the subsets are judged tocontain an error.

[0095] The signal quality detection unit 54 detects the signal qualitiesconcerning channels 0 to 3 according to an error signal provided as aresult of hard identification. If an output of the exclusive OR unit 53is not 0, the error correction unit 55 identifies a channel whose signalquality is detected to be the worst by the signal quality detection unit54. The exclusive OR of bits sent on the channels other than the channelis adopted as a bit sent on the channel. The bit is used to performerror correction.

[0096] The soft decision unit 56 performs soft identification on sixbits sent from the FFT unit 50. In this case, when the locations ofsignal elements other than the signal elements representing two bits arediscussed, the distance between the signal elements is extended. Thismeans that an error in identifying the signal elements is minimized.Data items sent on channels 0 to 2 are provided as 8-bit received dataitems, and data sent on channel 3 and containing error correction bitsis provided as 6-bit received data. The other channels are also groupedin fours, or in other words, divided into groups of channels 4 to 7,channels 8 to 11, channels 12 to 15, channels 16 to 19, channels 20 to23, channels 24 to 27, and channels 28 to 31. The aforesaid errorcorrection is performed group by group. This obviates the necessity ofperforming error correction on all bits and minimizes an error inidentifying data.

[0097]FIGS. 20A to 20D show the presentations of eye pattern views (20A)and (20C) and spectra (20B) and (20D) of a signal transmitted at atransmission speed of 2 (bits)×20 (channels)×4.8 (k baud rate)=192(kbps) according to 4 levels QAM within a power line carriercommunication system. A tone burst of noises is superposed on the signalas observed in the spectra (20B) and (20D). The channels a, b, c, and daround the tone burst are shown in the eye pattern views (20A) and(20C). In the eye pattern views (20A) and (20C), the channels a, b, c,and d are each assigned to four signal elements impressed on a signalcomponent.

[0098] Referring to FIGS. 20A to 20D, (20A) and (20B) are concerned witha case where error correction is not performed. When the single toneburst observed in (20B) is superposed on the signal, channel b near thesingle tone burst is affected most adversely as seen from (20A). Thesignal elements spread. This results in an error in identification. Alimit of a range of signal-to-noise ratios within which occurrence of anerror is not detected is +2 dB.

[0099] Moreover, a single tone burst of noises higher than the singletone burst of noises observed in (20B) is superposed on a signal asobserved in (20D). The eye pattern view (20C) demonstrates the fact thatchannel b near the single tone burst is affected most adversely. Thesignal elements spread more widely. Moreover, the signal elements sentover channel d also spread. Error correction is therefore performed.Occurrence of an error is not detected by this stage visualized as (20D)and (20C). The error correcting means employed in the present inventionand described in conjunction with FIG. 2 and FIG. 3 can be adapted tothis case. A limit of a range of signal-to-noise ratios within whichoccurrence of an error is not detected is −22 dB. Namely, when errorcorrection is performed according to the embodiment of the presentinvention, the efficiency in error correction improves to an extentequivalent to a signal-to-noise ratio of 24 dB.

[0100]FIGS. 21E to 21H show eye pattern views (21E) and (21G) andspectra (21F) and (21H) of signals transmitted at a transmission speedof 8 (bits)×30 (channels)×4.8 (k baud rate)=1,152 (kbps) according tothe 256-level QAM within a power line carrier communication system. Theeye pattern views (21E) and (21G) show channels a, b, c, and d near asingle tone burst similarly to the eye pattern views (20A) and (20C)shown in FIGS. 20A to 20D, wherein signal elements are grouped togetherand measured in order to express four levels instead of 256 levels. Asobserved in the spectrum (21F), even when the noise level of a singletone burst is low, signal elements sent on channel b spread overfrequencies. Without error correction, the limit of a range ofsignal-to-noise ratios within which occurrence of an error is notdetected is about +26 dB.

[0101] Moreover, the spectrum (21H) shows a single tone burst whosenoise level is higher than the noise level of the single tone burstshown in the spectrum (21F). The eye pattern view (21E) demonstratesthat signal elements sent on channel b are almost completely dispersed.Signal elements sent on channel d spread widely over frequencies.Namely, owing to the error correction in accordance with the embodimentof the present invention described in conjunction with FIG. 2 and FIG.3, the limit of a range of signal-to-noise ratios within whichoccurrence of an error is not detected has come to about +10 dB. Thus,the efficiency in error correction has improved to an extent equivalentto a signal-to-noise ratio of 16 dB.

[0102] The present invention is not limited to the aforesaid embodimentsbut can be implemented in various communication techniques includingradiocommunication. Moreover, the present invention can be implementedin identification of data reproduced from a recording medium that isequivalent to a data transmission line and that suffers from deformationin the waveform of a signal deriving from superposition of noises on thesignal. Moreover, the exclusive OR or modulo sum of data items sent onchannels other than an error correction channel is adopted as data senton the error correction channel. Alternatively, a parity bit may beused.

[0103] As described so far, according to the present invention, at leastone of a plurality of channels is used as an error correction channel.The exclusive OR or modulo sum of data items sent on the other channelsis adopted as data sent on the error correction channel. Data items senton a plurality of channels are identified, or data items reproduced overa plurality of channels are identified. It is checked if the exclusiveOR or modulo sum of identified data sent on an error correction channeland identified data items, which are sent on channels belonging to thesame group as the error correction channel does, is a predeterminedspecific value such as 0. If the exclusive OR or modulo sum is not thepredetermined specific value such as 0, it is judged that an error hasoccurred. The signal qualities concerning the channels are detected.Since the probability that data sent on a channel whose signal qualityis detected to have deteriorated contains an error is high, the dataitems sent on channels other than the channel whose signal quality hasdeteriorated are exclusive-ORed or modulo-added. The calculatedexclusive OR or modulo sum is replaced with the data sent on the channelwhose signal quality has deteriorated. Thus, error correction isachieved. The present invention is advantageous in that error correctioncan be reliably performed on data items sent on a plurality of channelsover a transmission medium or reproduced from a recording medium. Thetransmission medium over which high-level noises that are dispersed overa broad frequency band are superposed on a signal includes wirecommunication, radiocommunication, and optical communication.

[0104] Channels are divided into groups while being associated with thesides of the surfaces of a regular polyhedron. An exclusive OR iscalculated and adopted as error correction data. Consequently, errors indata items sent on a plurality of channels can be corrected. Moreover,channels associated with the sides of a plurality of surfaces adjoininga certain surface are grouped together. It is judged whether an error ispresent in each group. If an error is present, error correction isperformed on data items sent on a group of channels whose signalqualities have deteriorated. Thus, even if an error occurs on aplurality of channels simultaneously, the errors can be correctedreliably.

[0105] When each channel is assigned to a signal representing aplurality of bits, part of the bits is manipulated as a subset. Theexclusive OR of bits belonging to subsets is calculated and used toperform error correction. Signal elements representing the remainingbits are identified. Therefore, the identification can be achieved withthe interval between signal elements extended. Consequently, an error inthe identification can be minimized.

What is claimed is:
 1. An error correction device for correcting errorsin data items sent on a plurality of channels, comprising: an exclusiveOR unit for assuming that at least one of the plurality of channels isused as an error correction channel, calculating the exclusive OR ofdata items sent on the remaining channels, adopting the calculatedexclusive OR as data sent on the error correction channel, identifyingthe data items sent on the plurality of channels, and calculating theexclusive OR of the identified data items sent on the plurality ofchannels; a signal quality detection unit for detecting the signalqualities concerning the plurality of channels; and an identifying andprocessing unit for, if an output of said exclusive OR unit is apredetermined specific value, providing the identified data items senton the channels other than the error correction channel as they are, andfor, if the output of said exclusive OR unit is not the specific value,replacing the identified data, which is sent on the channel whose signalquality is detected to be the worst by said signal quality detectionunit, with the exclusive OR of the identified data items sent on thechannels other than the channel whose signal quality is the worst, andthen providing the resultant data.
 2. An error correction device forcorrecting errors in data items sent on a plurality of channels,comprising: an exclusive OR unit for assuming that channels areassociated with the sides of each surface of a regular polyhedron andthat a channel associated with one of the sides of each surface is usedas an error correction channel, calculating the exclusive OR of dataitems sent on channels associated with the remaining sides, adopting thecalculated exclusive OR as data sent on the error correction channel,identifying the data items sent on the plurality of channels, andcalculating the exclusive OR of the identified data items sent on theerror correction channel and the other channels associated with the samesurface as the error correction channel is; a signal quality detectionunit for detecting the signal qualities concerning the plurality ofchannels that is associated with the sides of a surface and groupedtogether; and an identifying and processing unit for, if an output ofsaid exclusive OR unit that calculates the exclusive OR of data itemssent on channels associated with a surface is a predetermined specificvalue, providing the identified data items, which are sent on thechannels other than the error correction channel associated with thesame surface as the channels are, as they are, and for, if the output ofsaid exclusive OR unit is not the specific value, replacing theidentified data, which is sent on the channel whose signal quality isdetected to be the worst by said signal quality detection unit, with theexclusive OR of the identified data items sent on the channels otherthan the channel whose signal quality is the worst, and providing theresultant data.
 3. An error correction device according to claim 2,wherein: assuming that channels are associated with the sides of eachsurface of a regular polyhedron and that channels associated with thesides of a surface and the sides of a plurality of surfaces adjoiningthe surface are grouped together, if the exclusive OR of identified dataitems sent on the channels associated with a surface is not thepredetermined specific value, said identifying and processing unitregards the same number of channels as the number of surfaces of theregular polyhedron, of which signal qualities are detected to havedeteriorated by said signal quality detection unit, as error channels;said identifying and processing unit then solves simultaneous equationswhose unknowns are data items assigned the error channels so as to thuscorrect the data items sent on the error channels.
 4. An errorcorrection device for correcting errors in data items sent on aplurality of channels, comprising: an exclusive OR unit for assumingthat each of a plurality of channels is assigned to a plurality of bits,that one bit or a plurality of bits out of the plurality of bits ismanipulated as a subset, that the plurality of channels is grouped inunits of a predetermined number of channels, and that one channel out ofeach group is used as an error correction channel, calculating theexclusive OR of subsets sent on the channels other than the errorcorrection channel, adopting the calculated exclusive OR as a subsetsent on the error correction channel, identifying the data items sent onthe plurality of channels, and calculating the exclusive OR of theidentified data items of the subsets sent on the channels belonging tothe same group; a signal quality detection unit for detecting the signalqualities concerning the channels, which belong to the same group,according to the subsets; an error correction unit for, if an output ofsaid exclusive OR unit is a predetermined specific value, providing theidentified data items of the subsets, which are sent on the channelsother than the error correction channel, as they are, and for, if theoutput is not the predetermined specific value, replacing the identifieddata, which is sent on the channel whose signal quality is detected tobe the worst by said signal quality detection unit, with the exclusiveOR of the identified data items of the subsets sent on the channelsother than the channel whose signal quality is the worst, and providingthe resultant data; and an identification unit for identifying bitsother than the subsets, and providing the bits together with the subsetshaving errors thereof corrected by said error correction unit.
 5. Anerror correction method for correcting errors in data items sent on aplurality of channels, comprising the steps of: assuming that at leastone of the plurality of channels is used as an error correction channel,calculating the exclusive OR of data items sent on the remainingchannels, adopting the calculated exclusive OR as data sent on the errorcorrection channel, and identifying the data items sent on the pluralityof channels; detecting the signal qualities concerning the channelsaccording to the identified data items; calculating the exclusive OR ofthe identified data items sent on the plurality of channels; and if thecalculated exclusive OR is a predetermined specific value, providing theidentified data items, which are sent on the channels other than theerror correction channel, as they are, and, if the calculated exclusiveOR is not the specific value, replacing the data, which is sent on thechannel whose signal quality is detected to be the worst as a result ofdetecting the signal qualities concerning the plurality of channels,with the calculated exclusive OR of the identified data items sent onthe channels other than the channel, and providing the resultant data.6. An error correction method for correcting errors in data items senton a plurality of channels, comprising the steps of: assuming thatchannels are associated with the sides of each surface of a regularpolyhedron and that a channel associated with one side of each surfaceis used as an error correction channel, calculating the exclusive OR ofdata items sent on the channels associated with the remaining sides,adopting the calculated exclusive OR as data sent on the errorcorrection channel, and identifying the data items sent on the pluralityof channels; detecting the signal qualities concerning the channelsaccording to the identified data items; and if the calculated exclusiveOR of the data items sent on the channels associated with a surface is apredetermined specific value, providing the identified data items senton the channels other than the error correction channel, which isassociated with the same surface as the channels are, as they are, and,if the calculated exclusive OR is not the specific value, replacing theidentified data, which is sent on the channel whose signal quality isdetected to be the worst as a result of detecting the signal qualitiesconcerning the channels, with the exclusive OR of the identified dataitems sent on the channels other than the channel, and providing theresultant data.
 7. An error correction method according to claim 6,wherein: assuming that channels are associated with the sides of eachsurface of a regular polyhedron and that channels associated with thesides of each surface and the sides of a plurality of surfaces adjoiningthe surface are grouped together, if the exclusive OR of identified dataitems sent on the channels associated with a surface is not thepredetermined specific value, the same number of channels as the numberof surfaces of the regular polyhedron, of which signal qualities aredetected to have deteriorated as a result of detecting the signalqualities concerning the channels, are regarded as error channels; andsimultaneous equations whose unknowns are data items assigned the errorchannels are solved in order to thus correct the data items sent on theerror channels.
 8. An error correction method according to claim 6,further comprising a step at which: if the exclusive OR of theidentified data items sent on the channels associated with a surface ofthe regular polyhedron is not the specific value, the same number ofchannels as the number of surfaces of the regular polyhedron, of whichsignal qualities are detected to have deteriorated as a result ofdetecting the signal qualities concerning the channels, are regarded aserror channels; a coefficient assigned to the error channel associatedwith the surface is set to 1; the error channels associated with othersurfaces that belong to the same group as the surface does aresuccessively selected and the coefficients assigned to the errorchannels are set to 0; and finally if the coefficients assigned to theerror channels associated with the same group of surfaces still assumeany values other than 0, the data items sent on the error channels arejudged to contain errors, and the errors are corrected.
 9. An errorcorrection method for correcting errors in data items sent on aplurality of channels, comprising the steps of: assuming that each of aplurality of channels is assigned to a plurality of bits, that one bitor a plurality of bits out of the plurality of bits is manipulated as asubset, and that at least one of the plurality of channels is used as anerror correction channel, calculating the exclusive OR of the subsetssent on the channels other than the error correction channel, adoptingthe calculated exclusive OR as a subset sent on the error correctionchannel, identifying the data items sent on the plurality of channels,and calculating the exclusive OR of the identified data items of thesubsets; detecting the signal qualities concerning the channelsaccording to the subsets; if the calculated exclusive OR is apredetermined specific value, providing the identified data items of thesubsets, which are sent on the channels other than the error correctionchannel, as they are, and if the calculated exclusive OR is not thepredetermined specific value, replacing the identified data, which issent on the channel whose signal quality is detected to be the worst asa result of detecting the signal qualities concerning the channels, withthe exclusive OR of the identified data items of the subsets sent on thechannels other than the channel whose signal quality is the worst, andproviding the resultant data; and identifying bits other than thesubsets, and providing the identified bits together with the subsets.10. An error correction method according to claim 9, further comprisinga step at which: assuming that the plurality of channels is divided intoa plurality of groups and that one channel out of each group is used asan error correction channel, the exclusive OR of subsets sent on thechannels other than the error correction channel, belonging to the samegroup as the channels do, is adopted as a subset sent on the errorcorrection channel belonging to the group; and an error in the subsetsent on the channel whose signal quality is detected to be the worst asa result of detecting the signal qualities concerning the channelsbelonging to the group is corrected according to the identified data ofthe subset sent on the error correction channel.